I've wondered about this myself.
Given what Kurt has said I'm not concerned about it separating once it's thoroughly mixed, especially in the freezer. But maybe there's a chance it arrives from the vendor less than perfectly mixed.
Problem is it arrives in a full bottle which makes shaking problematic, as someone pointed out above. So what do you do? Transfer it to a larger bottle and shake it? Pour it into a big measuring jug and stir it? Anything like that is going to add oxygen. I've heard people suggest using a bottle roller, which would work on a full bottle, but they aren't cheap.
Maybe it's
throwing the dice somewhat, but I decided not to worry about it. If there was a general problem with incompletely mixed nic solution it seems like we would have heard about it by now. And I can tell the difference between 6mg and 12mg
juice so I'm confident that if I mixed up some
juice with really high nic level I would notice.
We are talking about a small number of what seem like good professional vendors and I'm willing to trust their procedures.